At present, belt-driven starter generator (BSG) electric motors used in belt-driven start/stop mild hybrid systems of automobiles commonly adopt claw-pole electric motors for both driving and power generation. A claw-pole electric motor mainly consists of a casing, a stator iron core connected with the casing, claw poles adjacent to and alternate with each other, a rotor magnetic yoke, an excitation winding, a rotary shaft, a carbon brush, and a copper slip ring and so on. In the claw-pole electric motor, the main magnetic flux is generated only by electrical excitation of the excitation winding, which leads to a very low magnetic flux density in the air gap between the stator and the rotor of the electric motor; and there is serious magnetic leakage between the adjacent and alternate claw poles, which causes insufficient output of the electric motor when the driving system operates at a low speed.